﻿using System;
using System.Collections.Generic;

class University
{
    private HashSet<int> facNumbers = new HashSet<int>();

    public int StudentsCount
    {
        get
        {
            return facNumbers.Count;
        }
    }

    public void AddStudent(Student studToAdd)
    {
        if (facNumbers.Contains(studToAdd.Fn))
        {
            throw new ArgumentException("Student with such number already exists");
        }
        facNumbers.Add(studToAdd.Fn);
    }
}

